The present invention relates to a measuring machine, in particular to a Coordinate Measuring Machine (CMM) or Vision Measuring Machine (VMM) based on a Delta Robot structure.
In a conventional 3-D coordinate measurement machine a probe head is supported for movement along three mutually perpendicular axes (in directions X, Y and Z). Thereby, the probe head can be guided to any arbitrary point in a measuring volume of the CMM/VMM, and the object is measurable with a measurement sensor (probe) carried by the probe head.
For measuring surface variations, various measurement principles are known: one is based on the use of ultrasonic sensors/ultrasonic transducers (called ultrasonic probes), another one is based on the use of tactile sensors (also called tactile probes) and even another one is based on the use of optical sensors (also called optical probes or cameras). For gauging the surface of a target object the optical or tactile probe is movably fixed at an articulated arm, as it is shown for a tactile probe i.e. in EP 2283311 A1, or at a portal, as it is shown for an optical probe i.e. in WO 2008/135530 A1, so that it can be moved over the surface of the target object in the three Cartesian directions x, y, z. A further possibility to move a probe over a target object is mounting the probe movable in z direction and placing the target object on a table movable in x- and y-direction. Still a further possibility to move a probe over a target object is shown in brochure “Equator 300 Mess-Systeme” of Renishaw, published in July 2011; wherein a tactile probe is mounted on the movable carrier platform of a delta robot. However, as the tactile probe of this measuring machine has to contact the surface of the target object, the possibilities that the delta structure provides with respect to acceleration and motion speed is not fully exploited. Also based on a delta robot structure is the vision measuring machine using an optical probe described in WO 2014/040937, which more likely is able to use the full measurement speed of the delta robot structure.
The main advantage of the delta robot structure is that it is a light-weight construction, able to move very fast and also showing very high accelerations. The main disadvantage is however, similar as with a portal CMM or an articulated arm measuring machine the required volume of such a delta robot structure. In FIG. 1 a known CMM 10 using a delta robot structure 12 with a camera 22 as an optical probe 4 is given for acquisition of small scale 3D (3 dimensional) information, i.e. in the area of quality control of workpieces 50, wherein small scale is in the range of mm down to nm. As it can be seen, the delta robot 12 comprises a static top supporting platform 14 fixed by an arrangement of horizontal levers 11 at a stationary side frame 13 with piles 13′. The piles 13′ of the side frame 13 are protruding perpendicular from a frame table 15. Three middle jointed 26 arms 16, 16′, 16″ extending from the static top supporting platform 14. The arms 16, 16′, 16″, often called kinematic chains, are connected with their first end to the static top supporting platform 14 by means of first universal joints 24 and connected with their second end by means of third joints 28 to an end effector 18 supporting the probe 4. The end effector 18 is often built in form of a triangular or circular platform. The arms 16, 16′, 16″ are made of lightweight composite material and are driven by actuators (not shown) located in the static top supporting platform 14, controlled by a control unit 38. The control unit 38 can also be configured to serve as an analysing unit, but analysing of the data collected by the probe can also be analysed by an external computer like unit.
As the arms 16, 16′, 16″ are made of a light composite material the moving parts of the delta robot have a small inertia. This allows for very high accelerations and very fast movement, which outclasses by far those realizable by a portal machine or an articulated arm. But delta robots are highly sensitive with respect to temperature fluctuation and to strong vibrations during fast movement and fast acceleration/deceleration actions, caused by their lightweight construction. For measurements of high accuracy, the localisation of the end effector determined by the angle encoders 66 (FIG. 2) in the joints 24, 26, 28 is often not sufficient. Therefore, CMM 10 with a delta robot structure has been developed with a global measuring system 30 provided with stationary cameras 32 watching one or more markings 20, positioned at the arms 16, 16′, 16″ and/or at the end effector 18, from different perspective so that a precise determination of the current location of the end effector 18 can be given at any time. Nowadays the degree of freedom (DOF) of the end effector 18 of a delta robot had been extended from pure translation (movement in parallelograms only with 3 degrees of freedom (3DOF: translation in the x-, y- or z-direction and vectors thereof)) up to 6DOF, additionally allowing the end effector 18 rotational movements of maximum about 360° around those axis resulting in yawing, rolling, pitching.
As it can be seen from FIG. 1 the volume occupied by the delta robot 12 is considerable larger than the measuring volume, wherein the measuring volume is defined by the coverage of the movement of the end effector 18 with its probe 4; or with other words:
the measuring volume is the volume, which can be covered by the movement of the end effector 18 with the probe 4. In addition to the measuring volume a delta robot structure usually needs space for the top static supporting platform 14 and for the frame 15, 13, 11 which holds the top static supporting platform 14. This frame 11, 13, 15 adds volume to the actual measuring volume and also reduces accessibility to the measuring volume.
As in industrial environments utility space and used volume are important criteria, the efforts are directed to minimise the required utility space and the required volume.